The electrolysis of seawater or other dilute aqueous solutions of sodium chloride with consequent generation of active chlorine, i.e. of a mixture of hypochlorite and other oxidizing species, finds several applications in the industry which take advantage of the biocidal and disinfecting properties of the product. An application of particular interest is the biocide treatment of ballast water employed in the nautical field. As it is known, in fact, the ballasting and balancing procedures performed on vessels when they are displaced or in the course of loading and unloading operations in harbours are carried out on an amount of seawater stored in dedicated compartments, which are emptied or filled accordingly. To avoid contaminating the discharge areas with biological species coming from different environments it is compulsory to carry out a preventive biocide treatment of the water to discharge, eliminating in particular all sorts of microorganisms. Formulations of anodes based on noble metal oxide-coated titanium substrates suitable for generating of active chlorine from sodium chloride are known; however, the known formulations are generally characterized by a poor selectivity and efficiency when operating with electrolytes at low sodium chloride concentration, in particular at low temperatures as is the case of seawater. The anodic generation of active chlorine in unseparated electrolytic cells, typical of electrochlorination processes, is furthermore counterbalanced by a cathodic generation of hydrogen associated with a local increase in alkalinity, which favours the rapid scaling of the cathode surfaces. In this regard, a periodic washing of the cathodes with hydrochloric acid is commonly performed; such process poses some safety and environmental issues. An alternative solution consists of carrying out the electrolysis between two electrodes of identical formulation, alternately operating one as the anode and the other as the cathode and vice versa, by periodically reversing the applied polarity: in this way, a self-cleaning effect of the scaled cathodes is obtained under the effect of the local acidification induced by the subsequent anodic functioning. The noble metal oxide-coated titanium anodes of the prior art, however, have a tendency to get deactivated during the cathodic operation, thus negatively affecting the overall costs of the technology.
It would be then desirable to provide an electrode suitable for use in electrochlorination cells overcoming the drawbacks of the prior art.
In particular, it would be desirable to provide an electrode having a good selectivity in the generation of active chlorine from dilute solutions of sodium chloride even at low temperature, with a suitable duration for an industrial application even when subjected to periodic reversal of its polarity.